Most metals are reactive in electrolytic environments, such as the type of environment present in damp soil or water resulting in electrolytic corrosion. Electrolytic corrosion presents a particular problem for liquid storage tanks formed of metal, because corrosion can create holes, allowing the tanks to leak. Electrolytic corrosion is a particularly acute problem in metal liquid storage tanks such as the tanks used to store petroleum fuels at storage sites or service stations.
It's estimated that 3 to 5 million metal underground storage tanks are in service today. Failure or leakage of such tanks can have dramatic ramifications under current local, state and federal government regulations. In addition, storage tank failures due to corrosion and the resulting replacement costs dramatically impact the costs associated with their use and maintenance. Methods to increase the life of metal storage tanks and to decrease failures have a large impact on the operating and maintenance costs.
Electrolytic corrosion occurs on both the interior and exterior of fuel storage tanks. Basically, a corrosion cell is formed between different areas on the internal and external surfaces of the fuel storage tank. Variations in electrochemical activity or potential between one area on the interior or exterior surface of a tank and another area cause a corrosion cell to be formed between the areas. Although corrosion is most common on the exterior of a storage tank, it can also be a problem on the interior of the storage tank.
In order to minimize electrolytic corrosion problems, cathodic protection systems using either impressed current or galvanic protection are connected to the exterior of storage tanks. The galvanic anodes are formed of a metal that has a higher Electromotive Force than the material used to form the structure of the storage tank. Thus, current passes from the galvanic anode to the surface being protected, consuming the anode while preventing corrosion of the protected surface. Galvanic anodes used in tanks formed of ferrous materials such as steel are commonly formed of magnesium or zinc. There are a number of other anode materials that may be used, depending upon the application. The best anode material, and thus galvanic efficiency, depends upon the application.
Galvanic anodes are sacrificial elements that slowly corrode or are consumed in an electrolytic environment. Galvanic anodes may be consumed due to galvanic metal oxidation, which involves oxygen evolution and chlorine evolution. Because galvanic anodes are higher in Electromotive Force than the metal being protected, the corrosion or breakdown of the anode prevents the breakdown of the protected metal. In effect, the protected metal becomes a cathode of an electrolytic cell whose anode is formed by the sacrificial metal, i.e., "cathodic protection."
In cathodic protection systems using impressed current, small amounts of direct current are passed continuously from sacrificial anodes to the metallic structure to be protected. Controlling the amount of current passed between the anodes and the metallic surface halts the external loss of metal when the tank electrochemically reacts with its environment. Instead of the metal surface being protected from corroding, the sacrificial anode is corroded or consumed.
Cathodic protection of the exterior surface of a storage tank helps to prevent corrosion on only the exterior surface of the tank, but it does not prevent the interior surface of the storage tank from being corroded. Thus, to ensure that a storage tank does not fail due to interior corrosion, it would be beneficial to cathodically protect the interior surface of the tank as well as the exterior surface of the storage tank.
Galvanic anodes have not been commonly or effectively used inside storage tanks for a number of reasons. Sacrificial galvanic anodes release metal ions which can combine with water to form corrosive salts as the anodes break down. These corrosive salts can contaminate the liquid in a storage tank. If the liquid is refined fuel, the corrosive salts can make the fuel unusable for internal combustion engines. Specifically, corrosive salts can cause significant damage to the engine. Because the interior of a metal fuel storage tank is not cathodically protected, it is highly susceptible to interior corrosion, which can lead to fuel leakage, and thus costly environmental concerns.
As the petroleum industry becomes more environmentally conscious, there is increasing pressure to eliminate metal fuel storage tanks that may be susceptible to interior and exterior corrosion, and thus to possible petroleum leaks into the surrounding soil. This has led the petroleum industry to replace some underground metal fuel storage tanks at service stations and other locations with nonmetallic storage tanks formed of plastic or another polymerized or non-corrosive material. Nonmetallic fuel storage tanks are generally not as damage-tolerant or forgiving as metal fuel storage tanks, especially during earthquakes.
Because galvanic anodes must be replaced when the anode metal becomes sufficiently consumed, an anode within a storage tank should be easily replaceable. Further, in order to be effective, a galvanic anode must be positionable in the region where water accumulates in a storage tank, namely at the bottom of the tank. More specifically, because water is heavier than petroleum products, water tends to accumulate at the bottom of a storage tank underneath any fuel in the tank. In order for a galvanic anode to work efficiently, it should be located in direct contact with any water in the tank. Only by being located in the water will a low-resistance electrical circuit be created. If a low-resistance electrical circuit is not formed between the galvanic anode and the interior surface of the fuel tank, the galvanic anode will not effectively prevent the corrosion of the interior surface of the fuel tank.
Thus, there exists a need for cathodic protection systems that reduce or eliminate corrosion problems on the exterior and interior surfaces of metal fuel storage tanks such as those used at fuel storage sites or service stations. Such protection systems would allow fuel storage tanks formed of metal to be safely used without worry of corrosion, thus reducing the need for expensive and less damage tolerant plastic fuel storage tanks. As will be better understood from the following discussion, the invention provides a cathodic anode assembly that addresses some of the problems discussed above.